KITCHENER -- UPDATE: Region of Waterloo and union agree to resume discussions but talks break down.

About 700 Grand River Transit employees went on strike on Tuesday amidst ongoing labour talks, shutting down bus service across Waterloo Region.

The union representing the employees, Unifor Local 4304, says the top concern is workplace safety.

A tentative agreement was reached last week, but on Sunday, the union members voted against it, saying it didn't address violence that staff experience on the job.

"The incidents that do take place are verbal assaults, people have been spat on, they've had food and other things thrown at them," explains Tim Jewell, Local 4304 president.

The region says it's already offered to put in security barriers on the nearly 300 buses in the GRT fleet.

"We agreed early on that we would install barriers on all of the buses over the three-year term of this agreement, at a cost of something like $1.7 million," says Region of Waterloo Chief Administrative Officer Mike Murray.

But the union says it's not just about the barriers.

Jewell says response times for security or supervisors still range from six to 12 minutes.

Another sticking point may have been money: the region revealed some monetary terms in the agreement that was voted down on Monday.

The region says they offered six per cent raises for bus operators and 15 per cent for mechanics over the next three years.

"Money is great, it was one of the outstanding issues, but safety is the outstanding issue right now," Jewell says.

Buses stayed parked on Tuesday as members picketed outside multiple locations around the region, including Grand River Transit's main office.

The stoppage forced some local students to make back-up plans to get to class.

"It is very hectic and college has not been cancelled, so we are struggling a lot," one student told CTV.

Another said it was difficult as students because of low or non-existent income.

The LRT is still running, but if something goes wrong with the Ion, no shuttle buses are available. It can also be a challenge for those who rely on buses to connect to the LRT.